1. Make two equal sized patties. 2. Mix Chili sauce, ketsup, worcestershire, tarragon vinegar. Garlic, chili, oregano, salt and pepper, mix well. 3. Pour this sauce on one side of you hamburger. Add feta cheese. 4. Place other pattie on top and seal edges well. Squeeze a bit of lemon juice over pattie. 5. Fry hamburger pattie over high heat for about 2 minutes on each side, be careful this is quite a fragile pattie and may start to break apart, just pat it together as it cooks. 6. Lower heat to low add mushrooms to the pan cover the pan and continue cooking until pattie is well done about 10 minutes. 7. Toast bun, place pattie on bun and garnish hamburger with tomatoe, onion, Lettuce and mustard. Serve.